Aluminum and aluminum alloys have been made corrosion resistant by providing a non-chromium conversion coating thereon as disclosed in our U.S. Pat. Nos. 4,755,224; 4,711,667; 4,895,608 and 4,988,396. These patents are generally related to aluminum conversion coatings and although they broadly cover aluminum coatings they do not specifically disclose protecting anodized aluminum and/or aluminum alloys nor do these patents specifically show protecting aluminum and aluminum alloys with a non-chromium composition of permanganate and nitrate with a pH of less than 7.
Aluminum metal and it's alloys are in many cases anodized or given an oxide film of controlled thickness in order to enhance corrosion resistance, and or paint adhesion. Additional corrosion resistance is necessary as the anodized oxide film is quite porous and thus allows the underlining metal to be exposed to corrosive agents.
Additional corrosion resistance is supplied by a process known as sealing. One of the earliest and still most widely used process, seals the surface by hydrating the oxide film in boiling water or steam. (Setah, S. and Mitays, A. Proc. World Engineering Congress, Tokay 1929) This causes it to swell and thus close off the surface to corrosive agents. Still other early methods involved converting the oxide film to aluminum silicate solution U.S. Pat. No. 1,746,153 and British Patent 393,996 (1931) with a hot silicate, filling the pores with cobalt or nickel hydroxide, (Speiser, C. T., Electroplating and Metal Finishings, 1956, 9, No. 4, 109-16, 128) and for maximum corrosion resistance U.S. Pat. No. 1,946,162, hydrating the oxide film in order to seal in a hexavalent chromate solution.
Also, the prior art prior to our above patents was mainly concerned with using hexavalent chromium compositions having a pH of less than 7.